Push-Pull in the Preamp?

[rajones71]

Hi I'm a NEWBIE, but I was wondering why you never see a push-pull gain stage in the preamp. People always talk about preamp gain vs. power amp gain, and how for high-gain amps you can't beat the saturation of an overdriven push-pull poweramp. The downside, of course, is the high volume. Why can't you jimmy a eec83 or similar running in A/B in the preamp to get that rounded out/saturated wave form, then it wouldn't matter so much what kind of power amp you had because you could run the power amp clean, and still have normal preamp gain and the new A/B gain stage to choose from.

Anyone ever tried?

[MWJB]

Manufacturers usually go for P-P poweramps principally for the efficiency (you get more W out of 2 tubes PP than in SE), other factors include hum reduction and slightly different harmonic content.

A high gain amp will typically have more preamp stages, therefore more SE character as preamps are SE Class A cathode biased.

You could place the PI earlier in the preamp thus reducing the number of SE stages but this has cost implications as you then need to double up the number of tone controls & components, plus the more stages you have & the more components, the more likelyhood of imbalance (though this could feasibly be addressed later in the circuit).

This has been done...at least twice.

Then what do you do about reverb, or NFB if you want these features?

Would there be any point in doing this in a SE design, if you're just going to mix the 2 sides again at another SE preamp stage before going to the poweramp? Most SE amps are designed the way they are to be cheap.

Why would you necessarily want to run the poweramp clean? If all the distortion is coming from the preamp a lot of folks would find this harsh & buzzy.

[loudthud]

Many such things have been tried and if any of them worked, it would be well known. The thing that always gets left out is the interaction of the power amp and the speaker. Groove Tubes has a product (probably patented) where they went so far as to build a small power amp (~15W) and connect it to a simulated speaker load. I've heard it and there is still something missing.

Don't let the above discourage you. Learn all you can and try to build it.

[teemuk]

As an example of such designs [using push-pull stage in the preamp], Vox Valvetronix amps are first that come to my mind. Those also employ a feedback scheme that reflects the speaker load's impedance to the tube push-pull stage.

Now that I think of this more, if I remember right some Electar amps also used push-pull preamp stages - that time with transistors. Latest Peavey Transtube amplifiers employ a diode clipping circuit that operates similarly to push-pull stages and SRPP circuit (either tube or FET) is quite popular in footpedal "preamps". I'm sure that with a little searching you can find plenty of examples of these approaches.

[Enzo]

Hmmm. What PV amp uses clipping diodes? I've not seen that. They DO like to use pairs of diodes for noise reduction, but those are not clippers.

[teemuk]

Many Peavey amplifiers use diode clipping. Anyway, the specific circuit I was refering to is covered in US Patent 5,524,055 "Solid State Circuit For Emulating Tube Compression Effect". This is the clipper circuit used in conjuction with the Tube Dynamics circuit.

[CRU JONES]

Interesting idea! I think you should experiment.

The phase splitter on my Marshall sounds really good and if you look up info on the "long tailed splitter" and also stuff on differential amplifiers you should get some ideas.

One of the interesting things about differential amplifiers, is that the two tubes cancel out their nonlinearities... until one side runs out of head room. (Even so, they do sound good to me.)

The phase splitter that is used to drive a PP power amp set up in class A mode... So definitely try experimenting with biasing it colder (into class AB or B). In Class A mode you may use either plate's signal, but as you go further into class B (or AB),then each tube will need its plate signal combined together. This can be done with resistors. Or pot's ( if you like getting crazy).

Sorry if this is obvious!

[Joe Bee]

^^umm, I don't think it is possible to bias a long tailed pair in anything but class A, how is the second triode supposed to get a signal when the first stops conducting?

On the original question, I just came across this http://www.dogstar.dantimax.dk/tubestuf/4x4howit.htm
which you might find interesting. It utilizes a ush-pull preamp to be extra linar by sending feedback right to the first stage, but I am sure it could be adopted for dirty sounds aswell.

For the "cranked Plexi in a box" thing though I agree with the above posts, you need more than a saturated push pull stage. I would argue that a good dummy load should include at least one inductive element to mimic the impedance curve of a guitar speaker.

[rajones71]

Thanks all for the input/comments. I knew I couldn't be the first guy to think of that-but I didn't know enough about it to fully understand all the challenges in making it work. In fact, I've got a little more reading to do just to understand all the replies! Along those lines, I've read through Gerald Weber's latest book a couple times (okay, I've skimmed it a couple times), but was a little frustrated that he refers to his first couple books instead of going into detail sometimes. Any other recommended readings for a beginner?

-Thanks again everyone.

[CRU JONES]

after thinking about it... I realized I can't save my butt on that one. I think I will go learn how a diff amp works now... Bye!